Seems more like Hierarchical Triangular Meshing with hexagons. I had to use that for a project a few years back.

http://www.skyserver.org/htm/

https://arxiv.org/ftp/cs/papers/0701/0701164.pdf

No, the military grid is based on (a whole bunch of distinct skinny slices of) transverse Mercator projections, which don’t really line up properly at the seams, plus stereographic projections at the poles. It was largely designed for use with paper maps.

Discrete global grid systems based on hexagons are an idea which differ in most respects from that, except for the part about hierarchical coordinates (which can also be done using any arbitrary other map projection).